Ventilation questions

from e-mail:

I have read everything you have written and attended your lecture at ISGB last year. I am trying to work from your formulas but am not too sure.

I like what was done by this person using a feeder trough – I think a galvanized tub might also work This particular one comes out to 3.7 square ft face

If I use one of these

My calculation comes out to about 460 CFM (3.7 x 125)

What would happen if I used a 800 CFM???

I would have approximately 10 ft of duct with 2 bends – I thought 6 inch duct would work but I can use 8 inch.

The 465 CFM fan concerns me because it flanges out to 4 inches – Is this a problem?

My answer:

Bridgit —

The tub works just fine, no worries at all, in fact it is an excellent idea!

I am also not concerned about the outlet size of the fan, that really doesn’t matter at all. The outlet duct can be mounted on what’s called a “duct plug”.

The “duct plug” with fan attached is then mounted on your duct and you are in business.

Here’s the numbers for your proposed system:

460 CFM

6″ duct = 0.17″ static pressure
8″ duct = 0.03″ static pressure

800 CFM

6″ duct = 1.01″ static pressure
8″ duct = 0.14″ static pressure

As you can see, using 6″ duct is not really a good idea, especially if you choose to go with the 800 CFM fan.

The 125 CFM per square foot “rule of thumb” is used as a starting place for overhead hoods. I like to stay within about 10-15% of that number, especially for smaller torches like the Nortel Minor etc. Your upper end should be no more than about 530 CFM or so.

Keep in mind that by upping the CFM on the fan, your fresh air requirements will go up as well — which is the major reason I like to stay close to the design parameters for the fan. A well-designed and constructed system will not need more CFM, so there’s no reason to put excess capacity, especially if the system is going to be used for only one torch with one hood.


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